The technical field of the invention is photolithography and, in particular, stereolithographic patterning of materials.
Stereolithographic patterning enables rapid prototyping of complicated three-dimensional structures. Parts are built up in a layer by layer manner typically from 3D computer representations with no tooling or mounting changes during the build operation. Until recently, most techniques concentrated on fabricating macroscopic components (i.e.  greater than 1 cm3) with limited resolutions (i.e.  greater than 1 mil, or 25 micrometers). Recently, there has been growing interest in merging microelectronics with mechanical structures to create miniaturized microelectromechanical systems (MEMS) such as waveguide structures, microfluidic systems, and sensors. MEMS can offer cost savings, in terms of size reduction and/or increased functionality, especially when arrays of devices are implemented. Lithographic patterning is well established on the micro domain; however, structures are generally limited to extrusions of two-dimensional patterns because of the planar nature of lithography. Stereolithographic patterning on the micro-domain offers a greater optimization of structural elements and increased flexibility in package design.
Stereolithographic techniques utilizing photosensitive polymers have primarily suffered from either (1) poor layer thickness control and limited lateral resolutions or (2) significant process complexity because of the need for intermediate plating and development steps to be performed on a layer by layer basis. The previous methods reported utilize: (1) solution polymerization, (2) a combination of photoresist patterning with plating techniques, or (3) photoresist laminates.
In solution polymerization, a focused laser beam is scanned in a vector-based fashion over a solution of low molecular weight acrylic or epoxy resin. Laser-induced radicals promote polymer-polymer linkage (crosslinking) building up larger chained molecules, which then precipitate out of solution. The process was principally designed for the fabrication of macroscale parts and does not readily scale to the finer feature sizes and layer thicknesses achieved using integrated circuit (IC)-based resists and coating techniques. In solution polymerization , the coating technique is suited for macroscopic parts where individual layers are on the order of 10-50 micrometer thick. The fabrication process involves repeatedly lowering an elevated platform to a given depth per cycle permitting fresh material to overcoat the previously defined layer. The viscous nature of the resins used in solution polymerization require excessive leveling times to coat layers less than several micrometers. Even with plates or wipers to assist in leveling each layer, the coating technique cannot match the precision and uniformity achieved with the coating techniques employed in integrated circuit processing. The ultimate lateral resolutions of the photosensitive polymers used in solution polymerization are also inherently poorer than the photosensitive polymers used in integrated circuit processing. Solution polymerization-based systems rely on chain propagation of reactive radicals to initiate further polymerization; the photogenerated radicals, in the liquid state, readily diffuse. Lateral resolution is, as a result, limited. In addition, support structures are necessary to pattern reentrant structures (regions with no physical supports underneath) which are difficult to remove in the micro-domain.
In a second method, photoresist patterning and electroplating steps used in integrated circuit processing are combined to fabricate metallic structures. This process requires performing development, plating, and planarization steps for each and every layer, thereby adding significant process complexity and reducing throughputs. The photoresist/electroplating combination has only been viable for the creation of structures with a minimal number of layers. The introduction of the additional processing steps is not prohibitive in such circumstances.
Structures have also been fabricated using laminates of photosensitive polymers. Adhesives have been used to attach multiple layers of Dupont Riston(copyright), a photopolymer material used to pattern printed circuit boards. The thickness of such laminates so fabricated have been on the order of 20 micrometers thick or greater; and it has been unclear whether the use of thinner layers is possible in view of manufacturing and handling constraints. Three-dimensional parts have been fabricated using a combination of repetitive exposure and plating techniques. Laminates of polymethyl methacrylate (PMMA) have also been employed having similar thicknesses. Such layers are attached using solvent bonding by melting a surface skin between the layers. In thin layers, this causes pronounced distortions in the previously exposed portions or, otherwise, adversely affects the development process
None of the above methods can take direct use of advancements in high resolution photoresists and coating techniques already developed for the patterning of integrated circuits without the extra complexity of intermediate plating and development steps. The methods summarized above have been, in part, designed around a universal constraint that is posed because of solvent intermixing between layers of photoimageable resists. Solvent intermixing between the layers washes out previously defined features, alters the dissolution characteristics of the photoresist, and may possibly also cause thickness non-uniformities during coating.
Methods have been developed in semiconductor processing for layering multiple layers of photosensitive material. However, these techniques have been limited to bilayer or trilayer schemes and the resultant composite structures are not readily extendible to stereolithographic patterning that typically require a multitude of layers. Known methods for placing multiple layers of photosensitive polymers utilize: (1) two separate layers with immiscible solvents or an immiscible barrier layer between miscible layers (e.g. top-surface antireflection coatings, contrast enhancement layers, and lift-off based processes), (2) crosslinking to render an underlayer insoluble to the solvent of the next layer (e.g. bottom-layer anti-reflection coatings or in bi- or trilayer schemes where one layer serves as a planarization layer over reflective topographies), or (3) a sufficiently weak solvent in the applied layer to limit penetration into an underlayer to fabricate undercut profiles. All these methods present limitations in stereolithographic patterning.
All these methods present limitations in stereolithographic patterning and suffer from one or more of the following problems: Application of one layer destroys the photoimaging properties of another layer or the photoimaging properties are purposely destroyed through crosslinking to prevent solvent penetration, different developers are required for each layer complicating the development process or more typically, one layer is completely soluble in the developer of the other layer leading to delamination, or the composite structure is only compatible with a single exposure.
In the first case, different materials are applied for each layer. The top layer is purposely designed to be completely soluble in the underlayer""s developer, or a different developer is required for each layer. In multiple layers of the composite structure, delamination would occur or different developers would be cycled on each layer complicating the development process. In the second case, the photoimaging properties of the resist are destroyed by crosslinking. Rather, a pattern imaged from the top surface is transferred through it in a second development stage, typically using a plasma based process. Again, each layer of the structure produced from this composite structure requires cycling between different development steps and significantly complicates the development process in multiple layers of this composite structure. Lastly, solvent weakening has been attempted in only a few limited cases. Different materials (or resists with different molecular weights) are used for each layer. The processing conditions used to apply the bottom layer either would destroy the photoimaging properties of the top layer preventing continued stacking of this composite structure or the process is only compatible with a single exposure for both layers.
A need exists for a method that overcomes the deficiencies of the aforementioned stereolithographic methods. A technique that leverages advancements in high resolution photoresists and coating techniques already developed for the patterning of integrated circuits without the extra complexity of intermediate plating and development steps would satisfy a long-felt need in the art.
Techniques to optimize the development process and increase patterning throughputs using simpler more cost-effective patterning solutions would greatly benefit adoption of the present technique.
Methods for patterning multilayer resists are provided. In one aspect of the invention, the method includes depositing a first layer of photoresist material from a first solution onto a substrate. A portion of the first layer is exposed to radiant energy. The first layer is treated creating a treated surface. Treating may occur before, after or during exposure. A second layer of photoresist material from a second solution is deposited atop the treated surface, the treated surface capable of inhibiting penetration of a solvent component of the second solution into the first layer. A portion of the second layer is exposed to radiant energy. The steps may be repeated until a multilayer resist having greater than two layers has been completed. The method may further include developing the layers in a single, final step to remove the exposed portions of each layer when the photoresist material comprises a positive resist. It may further include developing the layers in a single, final step to remove all but the exposed of each layer when the photoresist material comprises a negative resist. The method may further include heating the multilayer resist following exposing at least one portion. The photoresist material can include a one or more resins selected from phenol-formaldehydes, polyhydroxystyrenes and cyclic polyvinyls. The radiant energy can be laser radiation ranging in wavelength from about 4 nanometers to 520 nanometers. More preferably, from about 157 nm to about 465 nm. More specifically, the invention can be practiced with laser radiation sources such as helium-cadmium laser (operating at 325 nm or 442 nm), argon ion lasers (operating at 355, 257 or 248 nm), XeCl excimer lasers (operating at 308 nm), KrF (operating at 248 nm) and ArF excimer lasers (operating at 193 nm) and Fluorine lasers (operating at 157 nm), In additional embodiments, treating includes oxidation that may be accomplished using an agent selected from ozone, peroxide, oxygen plasma and combinations thereof The treated surface may have carbonyl or hydroxyl functionality. Treating may increases the hydrophilicity or the hydrophobicity of the treated surface. The solution may comprise a solvent that is a mixture of liquids.
In a further embodiment, a method of making a multilayer stereolithographic preform is provided. A base layer of photoresist is deposited from a base solution onto a substrate. The base layer is surface treated to inhibit penetration of solvent into the base layer. An overlayer of photoresist is deposited from a second solution, the second solution comprising solvent, the overlayer located atop the base layer to create a multilayer preform. Subsequently developing the preform to create a multilayer stereographic pattern may be performed in a single, final step. Surface treating and depositing may be repeated a number of times, each additional overlayer deposited atop a surface treated overlayer.
In another aspect of the invention, a method of efficiently patterning a multilayer photoresist preform is provided. A first layer of photoresist is deposited onto a substrate. First portions of the first layer are exposed to a first dose of radiant energy. A second layer of photoresist is deposited atop the first layer and second portions of the second layer are exposed to a second varied dose of radiant energy. Modulation of the second dose is a function of second portion locations that reside atop first portions; such second portion locations being interior portions. The dose is greater for interior portions than for other second portions. The term xe2x80x9cmodulationxe2x80x9d as used herein is intended to encompass any adjustment in time-integrated intensity or fluence. Exposing and depositing may be repeated a number of times, n, thereby creating an n-layer photoresist preform. Thus, in this aspect of the invention, the dose is modulated over different portions of a layer in stereolithographic patterning to preferentially enhance development within the interior of the structure to reduce total development times.
The present invention enables stereolithographic patterning using a single resist material and development step. The technique is compatible with resist materials which rely on chemically induced solubility changes from the photodecomposition of a sensitizing agent. These are the dominant resist materials used in semiconductor processing. Examples include conventional diazonaphthoquinone/phenol formaldehyde (DNQ/novolac) based resists, or acid-catalyzed poly(hydroxystyrene) or poly(vinyl-cyclic) systems. The resists are positive tone in nature, hence, exposed portions are removed in developer. The use of positive tone resists in stereolithography poses two additional challenges which to date have not been previously explored. First, new methods are required to optimize the development process in these positive tone resists. With negative tone resists, unexposed portions are soluble and removed in developer at a constant rate. However, in positive tone resists, the degree of solubility at a given developer normality depends on exposure dose. Since the resist is semitransparent at the given exposure wavelength, the dissolution rate decreases with depth within a layer.
To efficiently pattern large contiguous areas rapidly, a procedure has been developed using spot-size modulation of the focused laser beam to more efficiently pattern interior portions. Critical is portions at the perimeter are patterned at high resolutions. The spot-size is progressively increased towards the interior allowing a controlled transition to coarser spot-sizes without impacting the exposure dose in critical portions. Patterning times are significantly reduced since in effect shells are patterned. An algorithm is defined to subdivide a layer into different zones, determine the appropriate focused spot-sizes used for each zone, and define the laser scan trace within a zone to enable efficient patterning of broad areas in positive tone resists. The technique relies on the fact that the actual intensity distribution within the transitional regions and interior portions do not have to be as tightly controlled as in the vicinity of the perimeter. The technique vastly simplifies the stitching of non-rectangular beams with lateral variations in intensity and relaxes the tolerances on alignment between different size beams.
Current patterning techniques using vector based laser scanning tools are unable to achieve high patterning throughputs in positive tone resists. Vector scanning is an attractive option in maskless patterning in rapid prototyping in terms of cost and simplicity compared to projection based methods employing a spatial light modulator or raster based laser scanning. In these systems, a focused laser beam can be scanned across the surface in an arbitrary fashion. Due to bandwidth limitations of the scanners, vector scanning approaches are best suited for applications where either outlines of a part or shells in combination with internal web-like features can be used to render the part. Negative tone resists allow parts to be patterned with shells and internal web-like features since unexposed portions are soluble in developer. For instance, in 3D patterning systems employing solution polymerization, interior portions are filled with a honeycomb pattern. With positive tone resists, however, because of the tone reversal, there is no equivalent web-like internal fill since entire regions must be exposed to alter solubility. Techniques which have used variable size exposure areas to fill internal sections image a variable rectangular aperture using a projection system simplifying field stitching. In addition, the energy distribution at the surface is constant independent of slit size for focused widths well above the diffraction limits of the projection system. Spot size modulation techniques using a variable sized beams has not been previously reported in stereolithographic patterning and in fact no occurrences have been found in the general literature even for two dimensional patterning. Unlike a mechanical tool-bit which occupies a fixed volume in space, the focused laser beam is circular and generally has a spatially varying lateral (i.e. Gaussian or Airy disk) distribution. It is difficult to develop a robust algorithm for both defining the appropriate beam diameter and incremental distances between different size beams for arbitrary geometries. There is, for instance, no analogous Fourier transform for decomposing a two dimensional area into a summation of Gaussian beams. Also, the dose uniformity is sensitive to any misalignment between beams.